1. Field of the Invention
The present invention relates generally to image scanning devices and image forming apparatuses.
2. Description of the Related Art
Conventional image scanning devices may perform photoelectric conversion of light (reflection light) reflected from an original document using a photoelectric transducer, such as a charge coupled device (CCD) image sensor, to generate an image signal representing an image of the original and convert the image signal into digital image data by analog-to-digital (A/D) conversion. A dark current flows through a photoelectric transducer even when light is not incident on the photoelectric transducer. Accordingly, such an image scanning device typically performs black level correction (hereinafter, “black correction”) to eliminate an effect of the dark current produced in the photoelectric transducer.
This black correction is performed by obtaining black levels of pixels in a dark condition, in which a light source is off, and subtracting the obtained black levels, as correction data, from image data obtained by scanning an original document. It is inefficient to obtain black-level correction data by switching off the light source each time scanning is performed; this is because time to wait for light amount to be stabilized is required each time the light source is switched on for scanning. According to a technique for black correction disclosed in Japanese Patent Application Laid-open No. 2010-136164, pixel-by-pixel black-level correction data obtained before starting scanning is updated based on a black level of shaded (optical black (OPB)) pixels.
However, the conventional technique described above updates all of the pixels of the pixel-by-pixel black-level correction data obtained before starting scanning evenly based on the black level of the OPB pixels. Accordingly, there can be a case where black correction cannot be performed appropriately when, for instance, amounts of change in dark current vary from pixel to pixel due to temperature distribution in a chip of the photoelectric transducer and/or a manufacturing process of the photoelectric transducer.
In view of the above circumstance, there is a need for an image scanning device capable of performing black correction even when amounts of change in dark current vary from pixel to pixel and an image forming apparatus including the image scanning device.
It is an object of the present invention to at least partially solve the problem in the conventional technology.